Defense Secretary James Mattis, left, and Joint Chiefs Chairman Marine Gen. Joseph Dunford. Thomson Reuters US Defense Secretary Jim Mattis said on Friday the US-led coalition was now encircling ISIS fighters in their strongholds in a tactical shift before starting a military campaign to destroy them.

"By taking the time up front to surround these locations, instead of simply shoving them from one [location] to another and actually reinforcing them as they fall back ... we now take the time to surround them," Mattis said in response to a question about the shift.

Mattis also described the need to focus on foreign fighters who, he said, were more dangerous because of their ability to slip back to their homes outside of ISIS territory.

"Those foreign fighters are a threat, so by taking the time to deconflict, to surround, and then attack, we carry out the annihilation campaign so we don't simply transplant this problem from one location to another," Mattis told reporters.

When aksed about what "annihilation" for ISIS would look like on the ground, Mattis underscored the removal of the transnational dimension for the terror group, reducing it to a threat local security forces could deal with.

"We drive them down to a point where the locals can handle that and it's no longer a transregional, transnational threat," Mattis said. "So you've got to drive them down to a point that police can handle. Police can't handle a force that's driving tanks or using artillery or has thousands of fighters or mobile vehicles [that] allow them to range far and wide."

When asked about how the shift would affect the duration of the campaign against ISIS, Mattis said he didn't put timelines on forces in the field; rather, commanders on the ground would focus on the threats in front of them.

"They know our intent is the foreign fighters do not get out," he said. "I leave it to their skill, their cunning to carry that out."

Also speaking at the Pentagon on Friday, Chairman of the Joint Chiefs of Staff and Marine Gen. Joseph Dunford said strikes against Syrian government-backed fighters this week were a "force-protection measure," and would not happen again if US forces are not threatened.

(Reporting for Reuters by Phil Stewart and David Alexander; writing by Yara Bayoumy, editing by G Crosse)

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